Marine propulsion systems for large merchant vessels such as tankers and cargo ships are usually either of the diesel engine type, or of the gas turbine type. Over the years, much attention has been given to increasing the fuel efficiency of these marine propulsion systems, to the exclusion of the other fuel consuming power systems on the vessel.
Diesel engines have been made for marine propulsion which produce up to about 10,000-90,000 horsepower and which operate at thermal efficiencies up to 52% and higher. These engines typically have multiple, large diameter cylinders and operate at relatively low rotary speeds of 80 to 120 RPM. These engines can be connected to the vessel's propeller with a relatively simple power transmission system, and may even connect to the vessel's propeller without a mechanical gearbox.
The manufacturer of these diesel engines have spent considerable effort to make these engines operate at very high efficiencies. However, there is generally a very narrow set of operating parameters on these engines for peak efficiency. In order to minimize the fuel consumption of the vessel as it makes its voyage, the engine is designed such that it operates at its peak efficiency under normal sailing conditions. The engine's efficiency degrades rapidly as its operating conditions vary. For example, it would not be unusual for a diesel engine operating at 40% to 60% to rated power to have an efficiency of only 15% to 20%.
These large diesel engines also produce copious amounts of smoky exhaust, which has caused concern, especially at ports in cities with existing high levels of air pollution, such as Los Angeles and Houston. The problem has grown to the extent that some ports are beginning to limit the levels of smoke permitted, causing some shippers to reschedule delivery or divert to other ports. In spite of these problems, however, the diesel engine type marine propulsion systems are the prevalent propulsion plant for large merchant vessels.
Although used less frequently than diesel engines, gas turbine engines are also used as marine propulsion plants. These engines generally have two turbine sections; a gas generation turbine section and a power turbine section. The two sections are operatively coupled to form a gas turbine engine. More details on the design and control of gas turbine engines may be found in U.S. Pat. Nos. 3,601,989; 3,808,804; 3,993,912; 4,338,525; 4,602,478; and 5,553,448, all incorporated herein by reference. The power output turbine section of marine gas turbine engines usually operates at a fairly high RPM. A speed reduction transmission is generally required in merchant vessels with gas turbine engines to reduce output speed to the 80 to 120 RPM range. Gas turbine engines can be made to be very efficient, particularly when a topping cycle is implemented. Topping cycles are well known in the gas turbine industry, and are described or referred to in U.S. Pat. Nos. 3,591,313; 4,123,200; 4,274,811; 4,719,746; 4,796,595; 5,267,432; and 5,297,384, all incorporated herein by reference. A typical topping cycle utilizes a wave rotor convertor that operates as a result of a pressure wave process that takes place in the cells of the wave rotor convertor. The topping cycle wave rotor convertor increases the pressure ratio of the gas turbine, and consequently the overall cycle efficiency of the gas turbine engine.
Gas turbine engines are able to operate at about the same total high thermal efficiencies as diesel engines, in the 55% range. Unfortunately, similar to diesel propulsion engines, gas turbine propulsion engines are highly efficient in only a very narrow band of operating parameters. Gas turbine engines also suffer from the same dramatic degradation in efficiency when operated outside the optimal range. When the transmission requirements are factored in, the overall costs for typical marines gas turbine propulsion systems are generally similar to marine diesel engines.
The principal advantage of gas turbine engines is their ability to produce much more power than diesel engines, up to about 100,000-500,000 horsepower. Also, compared to diesel engines, gas turbine engines produce far less noxious exhaust emissions, and are much lighter in weight. For example, a 30,000 horsepower gas turbine propulsion plant weighs only about 60 tons. The very high horsepower to weight ratio makes gas turbine engines the engine of choice in naval vessels. Although some gas turbine engines are used in the merchant shipping industry, the use is not widespread.
Regardless of the propulsion system used, however, the merchant marine industry has a daunting task to manage the overall energy usage (i.e. fuel consumption) on the vessels. There are three distinct operating modes in merchant vessels that require energy management. The first is the voyage energy requirements, the second is the cargo operations energy requirements, and the third is the standby mode energy requirement. To date, manufacturers of marine propulsion systems have focused on the engine's efficiency during the voyage. However, due to energy requirements at partial loads in the vessel, if a more fuel-efficient engine replaces an old propulsion engine, it is possible for the overall energy efficiency of the vessel to decrease. When evaluating fuel efficiency, all operating modes of the vessel must be considered.
In order to effectively compete, merchant marine vessels must be able to transport many different types of cargo on the same ship at the same time. It would not be unusual, for example, to have one cargo requiring sub-zero refrigeration to be transported simultaneously with a cargo that required heating. Therefore, the energy requirements for providing the heating and/or cooling may vary considerably from trip to trip dependent on the kind of cargo.
During the voyage, some electric power may be generated directly from the propulsion engines or by steam turbine generators utilizing exhaust waste heat from the propulsion engines. Since the electrical power requirements may vary considerably from trip to trip, it is difficult to devise a propulsion engine driven generator that does not affect the engine's efficiency. In addition, during cargo loading and unloading, the propulsion engine is generally shutdown, and at these times additional electric power is often needed for cargo handling equipment from auxiliary electrical generators.
One way to address the varying electrical power demand is to install a number of small, inefficient auxiliary diesel engine powered electrical generator sets. The number of generators operating would vary according to electrical power usage. Since the electric power consumption may be considerable at times, the reduced efficiency of these generator sets can significantly increase operating costs. In addition, engine/generator maintenance and higher crew labor costs may also increase operating costs.
Another energy consumer, cargo heating, is relatively easy to perform during the voyage because waste heat from the propulsion engine exhaust is available to operate boilers for steam heat. However, during cargo operations, and during standby periods, the waste heat from the propulsion engine is not available. Still, the cargo's temperature must be maintained, and at times, additional heating of a cargo is required to unload it. To provide cargo and other heating when the propulsion engine is shutdown, a number of auxiliary steam boilers are utilized. As is well known, steam boilers are very inefficient, and the fuel costs to operate these boilers is significant.
There are numerous other energy consuming devices that are affected by the operating mode of the vessel. For example, compressed air is sometimes used in vast quantities during cargo handling operations. The problem in the marine shipping industry is that even though the propulsion engines can have high thermal efficiencies, when the total fuel consumption of a merchant vessel is evaluated, the overall thermal efficiency is much lower, effectively in the 20% to 30% range.
What is needed is a new more efficient means of providing efficient central power generation that will be highly efficient regardless of variations in the heating, electrical and propulsion requirements. The same power generation system would have the capability of providing propulsion power, electrical power, thermal energy, and other forms of power at high efficiency levels regardless of variations in the operating mode of the vessel, or in the various configurations of the vessel from trip to trip. In addition, it would be desirable to install the new power generation plant for marine vessels at costs lower than those presently available for marine power generation plants.